1. Field of the Invention
This invention relates to a process for increasing the hydrogen content of a synthesis gas, in particular increasing the hydrogen content of a synthesis gas generated from a carbonaceous feedstock.
2. Description of the Related Art
Synthesis gas, also termed syngas, comprising hydrogen and carbon oxides (CO and CO2) may be generated by a gasification of carbonaceous feedstocks such as coal, petroleum coke or other carbon-rich feedstocks using oxygen or air and steam at elevated temperature and pressure. Generally, the resulting synthesis gas is hydrogen deficient and to increase the concentration of hydrogen, it is necessary to subject the raw synthesis gas to the water-gas-shift reaction by passing it, in the presence of steam, over a suitable water-gas shift catalyst at elevated temperature and pressure. The CO2 that is formed may then be removed in a downstream gas washing unit to give a hydrogen rich product gas. The synthesis gas generally contains one or more sulphur compounds and so must be processed using sulphur-tolerant catalysts, known as “sour shift” catalysts. The reaction may be depicted as follows;H2O+COH2+CO2 
This reaction is exothermic, and conventionally it has been allowed to run adiabatically, with control of the exit temperature governed by feed gas inlet temperature and composition.
Furthermore, where it is required that only fractional shift conversion is needed to achieve a target gas composition, this is conventionally achieved by by-passing some of the synthesis gas around the reactor.
Side reactions can occur, particularly methanation, which is usually undesirable. To avoid this, the shift reaction requires considerable amounts of steam to be added to ensure the desired synthesis gas composition is obtained with minimum formation of additional methane. The cost of generating steam can be considerable and therefore there is a desire to reduce the steam addition where possible.
WO2010/106148 discloses a process to prepare a hydrogen rich gas mixture from a halogen containing gas mixture comprising hydrogen and at least 50 vol. % carbon monoxide, on a dry basis, by contacting the halogen containing gas mixture with water having a temperature of between 150 and 250 DEG C. to obtain a gas mixture poor in halogen and having a steam to carbon monoxide molar ratio of between 0.2:1 and 0.9:1 and subjecting said gas mixture poor in halogen to a water-gas shift reaction wherein part or all of the carbon monoxide is converted with the steam to hydrogen and carbon dioxide in the presence of a catalyst as present in one fixed bed reactor or in a series of more than one fixed bed reactors and wherein the temperature of the gas mixture as it enters the reactor or reactors is between 190 and 230 degrees C. The space velocity in the water-gas shift reactor is preferably between 6000-9000 h−1. In the single Example a space velocity of 8000 hr−1 was used. Because this process operates at a low steam to CO ratio and at low inlet temperature it is limited in utility to certain types of gasifier and requires a relatively high catalyst volume. Therefore there is a need for a process operating at a low steam to CO ratio that requires less catalyst and which has broader utility.
WO2010/013026 discloses a process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, comprising the steps of (i) heating the synthesis gas and (ii) passing at least part of the heated synthesis gas and steam through a reactor containing a sour shift catalyst, wherein the synthesis gas is heated by passing it through a plurality of tubes disposed within said catalyst in a direction co-current to the flow of said synthesis gas through the catalyst. The resulting synthesis gas may be passed to one or more additional reactors containing sour shift catalyst to maximise the yield of hydrogen production, or used for methanol production, for the Fischer-Tropsch synthesis of liquid hydrocarbons or for the production of synthetic natural gas. While effective, we have found that in some cases with a cooled first shift reactor that the catalyst temperature profile may be too high, leading to undesirable side-reactions.